Specific pattern of check

[evgenybozhko2]

I want check the next: in field user can input characters, numbers or empty field. How I can implement that ?? #

[vekexasia]

 <com.andreabaccega.widget.FormEditText
           whatever:testType="alphaNumeric"
           whatever:emptyAllowed="true"
           android:id="@+id/et_firstname"
           android:layout_width="match_parent"
           android:layout_height="wrap_content"
           />